Signal information received by a radar system frequently includes unwanted echoes reflected from stationary or slowly moving reflectors such as the ground or sea, or from wind driven rain or chaff. The unwanted echoes obscure desired signals--such as those reflected from a moving target. The desired signals corresponding to a moving target usually vary quickly with time. The unwanted signals corresponding to sea clutter and wind driven rain or chaff vary slowly with time. This difference can be exploited to eliminate the unwanted signals since data signals corresponding to quickly varying signals are uncorrelated as a function of time whereas slowly varying signals are correlated with time. In other words, stationary reflectors yield return echoes that include frequency components which vary more slowly than the frequency components of the desired moving target. Therefore, unwanted signals are reduced or eliminated by separating the received signals into correlated and uncorrelated components and then by filtering out the correlated components.
To this end, various systems and techniques have been devised which filter unwanted signals by isolating the correlated components of a received signal and then canceling the correlated components from the received signal. One such system is a Gram-Schmidt canceller which receives a main signal and a plurality of auxiliary signals all corresponding to the same target object. The Gram-Schmidt canceller correlates each auxiliary signal with the main signal, generates weighting factors commensurate with the degree of correlation of the signals, and uses the weighting factors to eliminate the correlated components from the main signal to thereby yield one filtered output signal. Each additional data set is processed by the Gram-Schmidt canceller and new weighting factors are generated appropriate to the new data set. Such conventional systems are described in more detail below when comparing these systems with the present invention.
Systems using a Gram-Schmidt canceller or a similar technique also include those disclosed in the following references: U.S. Statutory Invention H92 (Kretschmer et al) which discloses a moving target indicator system incorporating a Gram Schmidt processor for reducing clutter in the signal; U.S. Statutory Invention H14 (Lewis) which discloses a moving target indicator system including an adaptive noise canceller using sliding-window derived weights; U.S. Pat. No. 4,688,187 (McWhirter) which discloses a Gram-Schmidt algorithm for applying linear constraints (weighting factors) to data inputs; U.S. Pat. No. 4,652,881 which discloses a moving target indicator system using a Gram-Schmidt adaptive noise canceller to decorrelate auxiliary signals from a main signal; and U.S. Pat. No. 4,398,197 (Dillard) which discloses a digital sidelobe canceller using weighted coefficients in the noise reduction process.